Wells are drilled with a fluid driven motor affixed to the lower end of a drill pipe. Drilling fluid is pumped down through the drill pipe by pumps situated at the surface of the drill string. The drilling fluid pumped downhole through the drill pipe passes through the motor and for any given motor, there is an optimum flow rate along with minimum and maximum allowable flow rates. After passing through the motor, the drilling fluid passes through a drill bit or other cutting tool before passing back up the annulus of the wellbore around the drill string.
Downhole operations routinely encounter pump rate limitations due to surface pressures exceeding the capacity of the pumps or the tubing. High surface pressures can come from a variety of sources such as drilling fluid weight, frictional loss, well conditions and pump rate. This problem is exacerbated when the downhole tools being used create additional back pressure or have rate limitations. These tools, in addition to positive displacement motors, can include hydraulic tractors, and multi-lateral entry tools.
Substantial amounts of debris often need to be cleaned out of horizontal wells during drilling of bridge plugs following a hydraulic frac operation. This debris can include frac sand, pieces of plugs, and formation. Typically wiper trips are performed every one or two plugs in order to clean the horizontal section of the well. The speed of the wiper trips is usually dictated by the annular velocity of the fluid in the wellbore.
In certain instances, removal of the debris in the borehole annulus requires a drilling fluid flow rate greater than the maximum allowed flow rate through the motor and therefore there must be a means for diverting some of the drilling fluid from the bore of the drill string to the annulus at a point near, but just above, the motor or other downhole tool. This will prevent exceeding the maximum flow rate for the motor, while providing an adequate flow rate in the annulus to remove the debris.
Some tools are known for this and similar purposes which require the pumping of a ball downhole to block a passage in the fluid flow path, usually resulting in the shifting of some flow control device downhole to divert drilling fluid to the annulus. Such tools usually suffer from the disadvantage of not being returnable to full flow through the motor, in the event that reduced fluid flow becomes possible thereafter. Other such tools might employ a fracture disc or other release means, with these release means suffering from the same disadvantage of not being reversible. Consequently, there is a need for an improved tool which will reliably divert drilling fluid to the annulus to achieve increased pump rates and corresponding annular velocities of debris removal.